The proposed research will continue our investigations of the metabolism of leucine and its Alpha-ketoacid, Alpha-ketoisocaproate (KIC) in vivo. Preliminary studies in dogs have demonstrated a rapid 3-4 fold increase in the rate of leucine carbon oxidation during meal absorption; however, the regulatory events responsible for this increase in oxidation and their ultimate effects on both amino acid and protein metabolism is poorly understood. The purpose of these studies is to quantitate the interactions of hormones, dietary components and substrates on leucine metabolism and should provide new insights into the regulation of whole body protein synthesis, proteolysis and amino acid catabolism. Using [4,5-3H] leucine, [1-14C] KIC and a dual isotope modeling technique, rates of leucine and KIC interconversion, oxidation of leucine carbon, and proteolysis will be measured and rates of protein synthesis estimated in conscious dogs. The addition of [15N] leucine and [6,6,6-3H3] leucine to a chemically defined diet will allow the partition of endogenous and exogenous leucine entry, thereby permitting quantitation of the rates of endogenous proteolysis during meal absorption. Specifically, these studies will: 1) determine the changes in leucine metabolism and rates of endogenous proteolysis prior to and during the mixed meal absorption; 2) determine if dietary fat and carbohydrate and/or amino acids other than leucine alter leucine carbon oxidation; 3) establish whether the liver is the primary site of leucine transamination and oxidation using transorgan catheter techniques; 4) detemine whether the rate of leucine carbon oxidation is primarily dependent upon portal leucine and/or KIC concentrations. These studies will determine the factors which regulate leucinde metabolism and should lead to the development technique to investigate mechanisms whereby amino acid and protein metabolism are altered in diseased states.